The present invention relates to a hydraulic control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling pressure of oil in a hydraulic circuit for the transmission.
A continuously variable transmission with a torque converter having a lockup clutch is known. The transmission is controlled by a high line pressure of 30 to 40 kg/cm.sup.2 at maximum and the torque converter is controlled by a lower control pressure of 4 to 6 kg/cm.sup.2. The transmission is further provided with a multiple-disk friction clutch in a selector device and a transfer clutch for a four-wheel drive system. These clutches are independently operated by respective pressures of the oil. Accordingly, it is necessary to control the respective pressures with accuracy.
Japanese Patent Laid Open No. 57-161360 (U.S. Pat. No. 4,478,105) discloses a system in which the pressure of oil from an oil pump is adjusted by a line pressure control valve for controlling drive and driven pulleys of the transmission. The line pressure control valve produces a control pressure for controlling a torque converter, a lockup clutch, a lockup control valve, a clutch and a brake for a selector device.
However, in such a system, the line pressure is applied to the clutch or brake in the selector device through a manually operated valve. The line pressure is greatly varied in accordance with the engine torque, and moreover flow rate of the oil is varied in accordance with the transmission ratio, so that the clutch pressure varies. Since the oil pump must supply oil to various devices at different pressures, an oil pump having a large discharge capacity is required, and hence pumping loss increases.
During reverse driving, reaction forces of input and output torque are exerted on a reverse brake of the selector device. Accordingly, it is necessary to provide a reverse brake having a torque capacity twice as much as the input torque. Unless the torque capacity of the reverse brake is sufficient because of a limitation of space of the device, the actuating pressure must be increased for the reverse brake. However, the pressure is too high for a forward clutch of the selector device, which is undesirable.
Further in such a system, if an abnormality occurs such as the decrease of the line pressure, a reduced line pressure is supplied to the clutch. Thus, the actuating pressure is lacking, causing the clutch to burn.